For increased energy efficiency, modern homes tend to be “tighter,” with less loss of heating and cooling. As a result, moisture generated inside a home may not find a way out and moisture from outside, coming in through a leak, may end up trapped in the home. Moisture is involved in many housing durability, health, and performance problems. Excessive moisture in a house can lead to rot, mold, swelling, staining, dissolution, freeze-thaw damage, and termite attacks that damage building materials and impede building performance. An understanding of moisture content and movement in a building is critical if one is to provide good housing performance, and is particularly important for walls, roofs, windows, and basements. Owners, builders, subcontractors, inspectors, designers, regulatory agencies, and product manufacturers require such knowledge.